The present patent application relates to electrochromic devices. More particularly, the present patent application relates to electrochromic devices utilizing Fabry-Pérot resonance cavities to maximize the dynamic range between the bleached and colored states of the electrochromic device.
The field of electrochromics is extensive and has been developing over about the last forty years. In one application, an electrochromic coating is used for controlling the amount of light and heat passing through the window based on a user-controlled electrical potential that is applied across the optical stack of the electrochromic coating. Not only can an electrochromic coating reduce the amount of energy used for room heating and/or air conditioning, an electrochromic coating can also be used for providing privacy. By switching between a clear state having an optical transmission of about 60-80% and a colored state having an optical transmission of between 0.1-10%, both energy flow into a room through a window and privacy provided by the window can be controlled. The amount of glass used for various types of windows, such as skylights, aircraft windows, residential and commercial building windows, and automobile windows, is on the order of one billion square meters per year. Accordingly, the potential energy saving provided by electrochromic glazing is substantial. See, for example, Solar Energy Materials and Solar Cells, (1994) pp. 307-321.
Over the forty years that electrochromics have been developing, various structures for electrochromic devices have been proposed including, solution-phase electrochromic devices, solid-state electrochromic devices, gasochromic devices, and photochromic devices.
For example, a conventional electrochromic cell generally is structured as follows: a substrate, a transparent conductive layer, a counter electrode, an ion transport layer, an electrochromic layer, and a transparent conductive layer. Conventional cathodic materials, commonly referred to as “electrochromic electrodes,” have included tungsten oxide WO3 (most common), vanadium oxide V2O5, niobium oxide Nb2O5 and iridium oxide IrO2. Anodic materials, commonly referred to as “counter electrodes,” include nickel oxide NiO, tungsten-doped nickel oxide, and iridium oxide IrO2. The ion layer materials exhibit a poor electron conduction, but good ion conduction. Examples of ion layer materials include SiO2, TiO2, Al2O3, and Ta2O5.
Various types of transparent conducting thin films have been employed for the first and second transparent conducting layers, such as, indium tin oxide (ITO), which is the most commonly used material. Other thin oxide layers have also been used, such as fluorine-doped tin oxide, antimony-doped tin oxide, aluminum-doped zinc oxide, and fluorine-doped zinc oxide. Regardless which thin film is used, conductivities of less than about 20 Ohms/□ are needed in order to produce a uniform voltage between the two conductive layers across the conductive layers. Even lower conductivities than about 20 Ohms/□ are needed for large panes of glass measuring 3-4 feet across.
If a voltage of between 1-4 Volts is applied between the first and second transparent conducting layers, the following reactions take place. At the anode, the following reaction takes place:                Metal Oxide or Polymer or Organic Molecule (Colored)+xM++xe Metal Oxide or Polymer or Organic Molecule (Transparent).At the cathode, the following reaction takes place:        Metal Oxide or Polymer or Organic Molecule (Transparent)+xM++xe−Metal Oxide or Polymer or Organic Molecule (Colored).in which M is H+, Li+ or Na+, e is an electron, and x is an integer.        
Fabry-Pérot filters are well known and have been used in a variety of optical filter applications ranging from narrow- and wideband pass filters to colored films to winter-based solar-controlled window films. Winter-based solar-controlled window films have been found to be useful in northern climates where solar light and heat transmission desirably through a window from the outside while room heat is rejected back into a room. For example, U.S. Pat. No. 4,799,745 to Meyer et al. relates to a heat-reflecting composite film that is used for various window film constructions. More recently, U.S. Pat. No. 7,339,728 to Hartig discloses low-emissivity (low-e) coatings for use on glass windows that are effective for reflecting infrared radiation. In embodiment, a Hartig coating comprises silver reflectors that are separated about 56 nm to about 65 nm by a dielectric spacer—a space that is too small for accommodating a reliably functioning electrochromic device.
U.S. Pat. No. 5,757,537 to Ellis, Jr. et al. relates to an electrochromic device that utilizes optical tuning to minimize optical interference between layers of the electrochromic device and to maximize uniform optical transparency. In one disclosed embodiment, shown in FIG. 11 of Ellis, Jr. et al., two metal conductive layers formed from, for example, Ag, Al or Cu, sandwich the cathodic electrochromic (EC) layer, the ion conductor (IC) layer and the anodic counter electrode (CE) of an electrochromic device. That is, the electrochromic (EC) layer is in contact with one metal conductive layer and the counter electrode (CE) is in contact with the other metal conductive layer. According to Ellis, Jr. et al., for this embodiment to achieve maximum optical transmission in a chosen wavelength between about 400 nm and 650 nm, the combined thicknesses of the three layers of the electrochromic device are constrained to be as thin as possible. If the three layers of the electrochromic device are selected to have indices of refraction of about 2.2, then their combined thicknesses, based on optical modeling, would be about 50 nm thick or about 300 nm thick.
The first modeled thickness of about 50 nm is near the practical lower limit for the thickness of an ion conductor (IC) film for an electrochromic device. Defects, such as pinholes and excessive electronic conductivity, begin to impair the reliable functioning of an electrochromic device when the thickness of the ion conductor (IC) layer is less than about 50 nm. Consequently, if the total thickness of an EC device is limited to about 50 nm, there is insufficient thickness to form a good ion conductor layer, and certainly there is insufficient thickness to make a reliably functioning electrochromic device. For the second modeled thickness of about 300 nm, if the IC layer is formed to have a thickness of at least about 50 nm, about 250 nm of thickness remains that can be apportioned between the EC and the CE layers, which presents a number of drawbacks. For example, when the metal conductor layers of the Fabry-Pérot filter are formed from silver, the silver is not protected from the EC layers and the lithium or proton ions. Moreover, the silver used for the metal conductor layers is pure silver, which makes the metal conductor layer susceptible to corrosion. Further, the dynamic range between the bleached and colored state, that is, the ratio of the spectral transmittance of the bleached state to the colored state, of the electrochromic device is limited because the EC and CE layers are so relatively thin. Notably, U.S. Pat. No. 5,757,537 to Ellis, Jr. et al. does not disclose what the reflected and transmission values are in the colored state.
Additionally, this particular embodiment of Ellis, Jr. et al. is a monolithic structure in which the components are between two glass panes. That is, a structure in which there is no air space between the two panes of glass. Such a structure has the disadvantage of having a lower thermal insulation than a structure in which there is an airspace between the two panes of glass.